I am a beginner of CFD and I am very interested in SU2 for its high fidelity and amazing optimization module. However, I have little knowledge about the "adjoint-based optimization". I have read the user manual and the AIAA paper about SU2, the references are only some papers. If I want to learn adjoint method, how to acquire some basic materials about it?

I am a beginner of CFD and I am very interested in SU2 for its high fidelity and amazing optimization module. However, I have little knowledge about the "adjoint-based optimization". I have read the user manual and the AIAA paper about SU2, the references are only some papers. If I want to learn adjoint method, how to acquire some basic materials about it?